The present invention relates to communications methods and apparatus, and more particularly, to spread spectrum communications methods and apparatus.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a conventional terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone communication system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a Public Switched Telephone Network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to the PSTN 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as Frequency Division Multiple Access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity.
Modern digital wireless systems typically utilize different multiple access techniques such as Time Division Multiple Access (TDMA) and/or Code Division Multiple Access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using xe2x80x9cspread spectrumxe2x80x9d techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread-spectrum CDMA communications systems commonly use so-called xe2x80x9cdirect sequencexe2x80x9d (DS) spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
Typical transmit operations of such a system are illustrated in FIG. 3. Data streams from different users are subjected to various signal processing steps, such as error correction coding or interleaving, and spread using a combination of a user specific spreading code and a group-specific scrambling code. The coded data streams from the users are then combined, subjected to carrier modulation and transmitted as a composite signal in a communications medium.
A so-called RAKE receiver structure is commonly used to recover information corresponding to one of the user data streams. In a typical RAKE receiver, a received composite signal is typically correlated with a particular spreading sequence assigned to the receiver to produce a plurality of time-offset correlations, a respective one of which corresponds to an echo of a transmitted spread spectrum signal. The correlations are then combined in a weighted fashion, i.e., respective correlations are multiplied by respective weighting factors and then summed to produce a decision statistic. The correlations generally are performed in a plurality of correlating fingers in the RAKE receiver, wherein each finger is synchronized with a channel path. The output of all fingers are combined to allow an improvement in the overall signal-to-noise ratio of the received signal. The design and operation of RAKE receivers are well known to those having the skill in the art and need not be described further herein.
In order to maintain the RAKE receiver fingers synchronized with their respective channel paths, a delay searcher may be used to support the RAKE receiver. The delay searcher can continuously search for new channel paths and estimate their delays. These delays then are assigned to RAKE fingers. A delay searcher also is referred to herein as a xe2x80x9csearcherxe2x80x9d. A delay tracker also may be used to continuously track the delays assigned to the RAKE fingers, to thereby allow improved accuracy of the delay estimates made by the delay searcher.
An embodiment of a delay searcher is described in U.S. Pat. No. 6,157,820 to Sourour et al., entitled Pilot Strength Measurement and Multipath Delay Searcher for CDMA Receiver, and assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference and in UK Patent Serial No. GB 2318952 to Vilmue, entitled Fast Pilot Channel Acquisition Using Matched Filter in a CDMA Radiotelephone, the disclosure of which is hereby incorporated herein by reference. An embodiment of a delay tracker is described in U.S. application Ser. No. 09/005,580 to Sourour et al., entitled Method and Apparatus for Multipath Delay Estimation in Direct Sequence Spread Spectrum Communication Systems, filed Jan. 12, 1998 and assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference. Finally, interaction between a delay searcher and delay trackers is described in U.S. application Ser. No. 09/168,233 to Sourour et al., entitled Delay Searcher and Delay Tracker Interaction for New Delays Assignment to RAKE Fingers, filed Oct. 7, 1998, assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference.
A delay searcher generally scans a wide search window of possible channel delays. For each delay in the search window, a long correlation generally is performed. More specifically, a typical multipath fading channel may include three or four channel paths. A conventional CDMA searcher that complies with the cellular standard TIA/EIA/IS-95 entitled Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System may search for channel paths over a wide window of delays around the delay of the first arriving path in the channel. This window of delays may be referred to as the search window.
According to the TIA/EIA/IS-95 standard noted above, a base station transmits a pilot channel comprising a fixed data value modulated by the pilot code. The pilot code includes a pseudo-random sequence of chips. For each delay position in the search window, the searcher generally performs a correlation between the received signal and the pilot code. After correlations for all delays are completed, the delays with the highest correlation magnitude may be selected as the correlations that correspond to the multipaths in the channels. These delays then are provided to the RAKE receiver. The search process may be repeated continuously to detect new channel paths and/or reject disappearing channel paths. See for example, the above-cited, U.S. Pat. No. 6,157,820 to Sourour et al.
As described above, the delay searcher scans a wide search window of possible channel delays. For each delay in the search window, the searcher generally performs a long correlation. Unfortunately, the correlation process may consume excessive power in the receiver. When the receiver is a portable radiotelephone or other portable terminal, excessive power consumption can unduly decrease the operational time and/or the standby time of the radiotelephone or other portable terminal. The correlation process also may consume excessive time in the receiver, which can reduce the operational efficiency thereof.
It is therefore an object of the present invention to provide apparatus and methods for determining channel delays in a spread spectrum signal that is transmitted in a communications medium according to a spreading sequence.
It is another object of the present invention to provide improved delay searching methods and apparatus that perform correlations over a search window of possible channel delays.
It is still another object of the present invention to provide delay searching methods and apparatus that can reduce power consumption when performing correlations.
These and other objects may be provided, according to the present invention, by correlating a composite signal including a spread spectrum signal from a communications medium with a local spreading sequence. Correlation is performed for a first set of time-offsets between the composite received signal and the local spreading sequence according to a first correlation length. Further correlations for decreasingly smaller subsets of time-offsets are performed according to increasingly longer correlation lengths to produce a final subset of at least one time-offset of the composite signal with the local spreading sequence.
The present invention stems from the realization that the final goal of the searcher is to detect a small number, such as three or four, channel delays, by selecting a small number of the largest correlation magnitudes in the search window. The remaining correlations are rejected. However, conventional searchers typically perform correlations between the received spread spectrum pilot code and the local replica of the pilot code for all delays in the search window, with equal correlation length. Thus, for example, for a typical search window of sixty chips as defined in Section 6.5.2 of the TIA/EIA/IS-95 standard (Recommended Minimum Performance Standards for Dual-Mode Spread Spectrum Cellular and PCS Mobile Stations), and a search precision of two delay positions per chip, the search window may perform 120 correlations, from which three or four are eventually selected. A considerable amount of power may be consumed in performing fall correlations for candidate delays that eventually will be rejected. According to the present invention, the correlation length for delays that do not appear to be qualified candidates may be shortened. This can significantly reduce the time and/or power consumption in the search.
More specifically, the present invention can determine channel delays when a spread spectrum signal is transmitted through a multipath communications medium. The composite signal is correlated with the spreading sequence according to a first correlation length, to produce a plurality of correlations corresponding to a first set of time-offsets. Selected correlations of the plurality of first correlations are further correlated with the spreading sequence according to a second correlation length that may be longer than the first correlation length, to produce a plurality of second correlations corresponding to a second set of time-offsets which is a subset of the first set. At least one of the plurality of second time-offset correlations may be selected.
It will be understood that the selected ones of the plurality of first time-offset correlations of the composite signal may be selected based upon the highest correlations or may be selected based upon correlations that exceed a predetermined threshold. Other selection criteria, or combinations of criteria, also may be used. When thresholds are used, succeedingly greater thresholds preferably are used. It also will be understood that after further correlating as described above, still further correlating may be performed. For example, selected ones of the plurality of second time-offset correlations of the composite signal may be correlated with the spreading sequence according to a third correlation length that may be longer than the second correlation length, to produce a plurality of third time-offset correlations of a composite signal with the spreading sequence. Additional correlation iterations may be performed.
Apparatus (systems) for determining channel time delays in a spread spectrum signal transmitted in a communications medium according to a spreading sequence, according to the present invention, preferably include a receiver that receives a composite signal including the spread spectrum signal from the communications medium. A variable length accumulation searcher correlates the composite signal with the spreading sequence according to a first correlation length, to produce a plurality of first time-offset correlations of the composite signal with the spreading sequence. The variable length accumulation searcher further correlates decreasingly smaller subsets of the plurality of first time-offset correlations of the composite signal with the spreading sequence according to increasingly longer correlation lengths, to produce a final subset of at least one time-offset correlation of the composite signal with the spreading sequence. During each correlation, the variable length accumulation searcher may select the time-offset correlations based on a highest correlation, correlations that exceed a threshold and/or other criteria.
A RAKE receiver is responsive to at least one of the final subset of at least one time-offset correlation of the composite signal with the spreading sequence in order to recover information encoded in the spread spectrum signal. Accordingly, by providing variable length accumulation searchers, power and/or processing time can be saved by performing limited correlations for those delays that do not appear to be qualified candidates to be ultimately selected. Improved spread spectrum receivers and receiving methods thereby may be provided.